Heart malformations have been found to occur frequently in mice homozygous for a gene (iv) that causes situs inversus. It is proposed to delineate the kinds of malformations found and to study their pathogenesis. It is intended to use this mouse mutation as a model for specific human heart malformations, particularly endocardial cushion defects, transposition of the great arteries, and double outlet right ventricle. It is further proposed to investigate the similarity of the "iv syndrome" of the mouse to the human asplena and polysplena syndromes since results of preliminary work indicate that there are similar extracardiac as well as cardiac malformations. Finally it is proposed to continue detailed breeding studies of iv/iv mice to learn more about the pattern of inheritance of situs inversus and its associated malformations since again the mouse appears to be a good model for situs inversus in the human patient.